Incorrect. The vast majority of the airspace over the contiguous United States is controlled, though there is a lot of it where participation in ATC is not necessary for VFR flight. From 1,200 feet AGL up to 18,000 feet MSL you're in Class E, and from 18,000 to 60,000 you're in Class A. Above that you're in Class E again. In some places, usually over some un-towered airports, Class E will extend down to 700' AGL or down to the surface as marked on sectional charts. Class D airspace, as well as the center columns of C and B airspace, extend to the surface.

Class G airspace pretty much only exists below 1,200' AGL in most places, I think there are remote areas in the middle of the flyover states and Alaska where the Class E floor is higher because there's nothing there, but that may be changing with ADS-B and shit.

It is not mandatory to participate in air traffic control to fly in Class E airspace. Laymen tend to use "controlled airspace" to mean "off limits without permission" but that's not how that works; Restricted areas for example require clearance to enter but exist as a separate concept to the alphabet airspace system.

"Controlled airspace" means some part of the air traffic control system has coverage in that area and can provide traffic separation and sequencing for IFR flights. For VFR it's a little more complicated; in Class A airspace (high altitude en-route airspace) VFR flight is not allowed. Terminal airspace (Class B, C and D, found around airports) participation in ATC is required for all flights. ATC services in Class E airspace is optional for VFR and is on a "workload permitting" basis.

Descend and land. You’d have done that long before “starting to enter” restricted air space. Otherwise enjoy your fine and potentially losing your license once the FAA finds out.

It’s like not you can accidentally enter restricted air space. You know you’re going to be entering the airspace long before you get there so there is plenty of time to take action.

Flight planning.

You presume a scenario where you make this realization while you are in the air. In practice, you made this realization hours earlier, and adjusted your flight plan to avoid that problem. You selected a launch site that avoids putting you in that situation.

If I do find myself encroaching on controlled airspace, I could declare an emergency, inform the controller where I am so they can vector traffic away from me, and do what I need to do to get down safely. At the very least, I would expect to do a lot of paperwork, and possibly have my license suspended.

With Class B and C airspace, the boundary is altitude dependent. The outer ring of Class C is 10 miles from the airport. I can fly between 5 and 10 miles of Akron Canton Airport, I just can't climb above 2500' MSL while in that area. I generally plan my flights to stay outside 10 miles, with the understanding that I can just stay below their airspace if I happen to get too close.

Small point of grammar: Floating "over" controlled airspace means you are still outside of it. Airspace is 3 dimensional so in addition to having horizontal boundaries, it also has vertical boundaries. Class C airspace for example, which you find around semi-busy airports like Raleigh-Durham International, looks kind of like a quarter stacked on top of a penny, except the stack is 4000 feet tall and 10 miles in diameter. You remain outside of the Class C airspace if you fly directly below the outer "ring." Or if you fly directly above it. I've done both, though I usually make a habit of calling up the approach controller and requesting flight following so that they can talk to me if they need to ("me" being a licensed pilot flying Skyhawks or smaller).

If you are going to fly an ultralight aircraft, you should seek out and receive training about the national airspace system, learn how to read a sectional chart, read things like Part 91, etc. I would advise carrying an aviation COM radio and monitoring local CTAF frequencies.

If flying something like a free balloon, you should know the prevailing conditions before takeoff. If the wind is blowing in the direction of a no no place, just don't launch. Stay on the ground until conditions for safe and legal flight exist.